This invention relates in general to the formation of electrically-conductive ceramic compound coatings on substrates and, more particularly, to the direct ion mixed plasma deposition of such compounds onto organic substrates such as organic matrix composites.
A number of different methods have been developed for depositing materials, generally metals, in the form of particles or ions onto a substrate surface to form an adherent, uniform coating. Among these are thermal deposition, cathode sputtering and chemical vapor deposition. While useful in particular applications, these methods suffer from several problems, including a tendency to coat other system surfaces than the substrate with the material being deposited, requiring frequent cleaning, contamination problems when the coating material is changed and a waste of often expensive coating material. Generally, these processes require that the substrate surface be heated to a very high temperature which often damages the substrate material, especially when the substrate is an organic material or an organic matrix composite material. The high deposition temperatures also lead to thermal stresses that may cause coating delamination.
Vacuum arc deposition has a number of advantages for coating difficult materials, such as refractory metals, onto substrate. Vacuum arc deposition involves establishing of an arc, in a vacuum, between a cathode formed from the coating material and an anode, which results in the production of a plasma of the cathode material suitable for coating. The process does not involve gases, making control of deposition rate easier and simplifies changing coating materials. Typical vacuum arc deposition systems are described in U.S. Pat. Nos. 3,566,185, 3,836,451 and 4,714,860. Vacuum arc deposition, sometimes referred to as cathodic arc deposition, is used commercially, typically to produce titanium nitride coatings on tooling.
A number of problems remain, however, which limit the use of vacuum arc deposition. Coatings often suffer from adherence and low density problems, particularly when an organic matrix composite material is used as the substrate. Difficulties are often encountered in obtaining a desired coating composition where ceramic materials, such as electrically-conductive ceramic compounds (e.g., borides, nitrides or carbides) are being applied.
Thus, there is a continuing need for improved methods and apparatus for forming uniform, adherent coatings from metal compounds, in particular on organic matrix composite substrates.